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Measurement of the unattached radon decay products with an annular diffusion channel battery.


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N. Michielsen, V. Voisin, G. Tymen, Natural radiation environment, 20-24 may 2002, Grèce.

Résumé

About one-half of the effective dose from natural sources is estimated to be delivered by inhalation of the short-lived radon decay products (UNSCEAR, 1999). Assessment of the dose delivered to the respiratory tract requires knowledge of the precise size distribution of the inhaled particles. Current models of deposition in the human respiratory tract show that the radiation dose per unit intake is greater for the radon decay products associated with the so-called "unattached" or nanometer fraction than for those associated with ambient aerosol. Besides, the diffusion characteristics of the unattached fraction influence its attachment to ambient aerosol and deposition on surfaces, which are the basic processes used in room model to calculate the activity balance of radon decay products in indoor air. (Gouronnec et al., 1996). Most commonly used instruments for measuring the unattached radon decay products and their diffusional properties are based on wire screen method. But this method has two major drawbacks: collection of attached radon decay products on the screen and losses by recoil. A new technique using an Annular Diffusion Channel (ADC) (Tymen et al., 1999), modified to allow continuous measurements of the unattached fraction (Huet et al., 2001), avoid these two drawbacks and furthermore presents a better selective geometry. We designed a diffusion battery based on this method for 0.3 to 5 nm particle diameter size range (i.e. particle diffusion coefficient between 0.2 to 22 mm2.s-1). The aims of the presentation are to describe the diffusion battery in details and to show the advantages. The ADC diffusion battery consists of six units, i.e. five ADCs of different length plus a reference unit, all operated in parallel (Figure 1). The sampled air is drawn through the ADC where diffusive particles are deposited; the remaining ones are collected onto a membrane filter ( Millipore 0.8 µm type AAWP). The alpha particles emitted by the 218Po and 214Po collected, or formed on the filter, are detected by an alpha PIPS detector (Canberra 450) placed in the inner tube of the ADC opposite the filter. Optimum ADC design and operating parameters were obtained using the collection efficiency determined by Kerouanton et al. (1996). The filter-detector distance and the annular channel width were set respectively to 4 mm and 2 mm. Table 1 presents the other parameters where D50 and d50 are respectively the diffusion coefficient and the diameter of the particle for a 50% collection efficiency. Figure 1: Schematic diagram of one ADC and the reference unit. Length (cm) Flow rate (l.min-1) D50 (mm2.s-1) d50 (nm) ADC1 2.5 13 11.5 0.46 ADC2 4.5 13 6.4 0.7 ADC3 10 13 2.9 1.2 ADC4 25 13 1.1 2.1 ADC5 30 5 0.37 3.7 Reference unit - 13 - - Table 1: Operating parameters of the ADC diffusion battery.We use a non-linear inversion method to process the activity data measured on each unit and to reconstruct the size and the diffusion coefficient distribution. First measurements with the ADC battery are presented by Vargas et al. (2002).ReferencesGouronnec A.M., Goutelard F., Montassier N., Boulaud D., Renoux A., Tymen G., Aerosol Sci. Technol., 25, 73-89, 1996.Huet C., Tymen G. and Boulaud D., Aerosol Sci. Technol., 35, 553-563, 2001.Kerouanton D., Tymen G. and Boulaud D., J. Aerosol Sci., 27, 345-349, 1996.Tymen G., Kerouanton D., Huet C. and Boulaud D., J. Aerosol Science 30 , 205-216, 1999.UNSCEAR (1999). Exposures from natural radiation sources,48 session of UNSCEAR, Vienne, 12 to 16 april 1999.Vargas A., Michielsen N., Le Moing C., Rio M., Tymen G. and Ortega X., Abstracts of the VII Congress of Natural Radiation Environment, Athens (Greece), 2002. This work has been done with the collaboration of the Université de Bretagne occidentale, France.